This invention relates to methods for preparing a high-purity molybdenum (Mo) or tungsten (W) powder, and a high-purity molybdenum oxide or tungsten oxide powder, and more specifically to methods for preparing an Mo powder or W powder or oxides powder thereof having an extremely high purity in which an amount of impurities are lowered to an order of ppb, with ease and at a less cost, the impurities being represented by radioactive elements such as uranium and thorium; alkali metals such as sodium and potassium; and heavy metals such as iron and nickel.
Circuit lead wires of MOS-LSI elements, above all, 256K bits or more VLSI elements are usually formed by applying a sputtering process in which targets comprising Mo or W are employed. At this time, it is an essential requirement that the targets are high in purity.
For example, if an impurity such as uranium or thorium is present in the target material, the formed VLSI elements will deteriorate in the reliability of operation owing to a bad influence of .alpha. rays. For this reason, it is necessary that a tolerance of the radioactive element is on the order of ppb. Further, since an alkali metal such as sodium or potassium easily migrates in gate insulating membranes of the elements in order to decline interfacial properties of MOS, its tolerance must be on the order of several tens of ppb. Furthermore, since a heavy metal such as iron or nickel will cause a leak phenomenon at an interfacial junction, and its tolerance is also required to be on the order of several tens of ppb.
The above-mentioned targets are usually manufactured by sintering Mo or W. Accordingly, in order to obtain the high-purity Mo or W targets containing less impurities, an Mo powder or a W powder which will be used as a raw material, needless to say, must not contain such impurities as mentioned above, and in short it must be of an extremely high purity.
The Mo powder may be generally prepared by first dissolving an Mo ore in a solution of an alkali such as caustic soda or aqueous ammonia; adding nitric acid to the resulting sodium molybdate solution or ammonium molybdate solution to form molybdic acid; dissolving this molybdic aicd in aqueous ammonia; concentrating the solution under a reduced pressure by heating in order to precipitate the crystal of a molybdic acid; and reducing the precipitated crystal with hydrogen. In like manner, the W powder can be preared by preparing a tungstic acid and then reducing it with hydrogen.
However, these methods scarcely permit removing impurities substantially and thereby lowering their concentration to the order of ppb, and in particular, the removal of radioactive elements is impractical. In an improved technique of the above-mentioned methods of heightening the purity of the product, a recrystallization process is applied to the crystal of a molybdic acid or a tungstic acid.
Even in the case of the application of this recrystallization process, however, both the molybdic acid and the tungstic acid have a peculiar property of adsorbing impurities, and thus such a manner as mentioned above is too low an impurity removal efficiency to be practiced. In addition thereto, the recrystallization process takes a long period of time to carry out a purification treatment and requires intricate extra facilities, which fact will remarkably increase the costs of the Mo powder and the W powder.
For the enhancement of the purity of target materials, a purification method (an EB melting process) has been developed in which an electron beam is applied to the sintered bodies manufactured by the Mo or W powder. According to this method, alkali metals and heavy metals can be relatively easily eliminated, but uranium and thorium are difficult to remove, because vapor pressures of uranium and thorium are close to those of Mo and W. In consequence, the EB melting process is not considered to be a satisfactory means for the high purification.